ACR X HAD LTSS FROM B757 ON APCH. SYS ERROR.

Narrative

ACR X; IAH TO DFW. DFW LNDG S. 1) X VECTORED INTO APPROX 8-10 MI FINAL TO RWY 18R; HANDED OVER TO DFW TWR. (APCH HAD TOLD US TO MAINTAIN 170 KIAS UNTIL MARKER.) 2) TURN ADVISED US OF WX (HVY TSTM) MOVING ACROSS FIELD; S TO N; RAIN WAS ABOUT HALFWAY DOWN RWY MOVING N; WE HAD BEEN CLRED TO LAND. 3) TWR THEN ADVISED US AN ACR Y; 757 HAD BEEN TURNED IN FRONT OF US FOR THE SAME RWY. 4) THE TCASII SHOWED THE 757 TO BE 2 1/2 TO 3 MI IN FRONT OF US. I GOT A VISUAL ON THE 757; IT WAS ABOUT 3 MI AHEAD. 5) I REQUESTED RWY 18L AND WAS CLRED TO LAND ON RWY 18L. THE 757 CONTINUED FOR RWY 18R. 6) THE 'WALL OF RAIN' HAD MOVED TO ABOUT THE 1000 FT MARKER ON BOTH RWYS. THE LNDG WAS UNEVENTFUL (VISIBILITY WAS LOST ON THE ROLLOUT DUE TO HVY RAIN). DFW CTLRS - IT IS COMMON KNOWLEDGE; GIVE PREFERENTIAL TREATMENT TO ACR. THIS PREFERENTIAL TREATMENT COMPROMISED THE SAFETY OF MY FLT BY PLACING A B-757 2 1/2 MI IN FRONT OF MY ACFT WHILE CONDUCTING AN APCH INTO RAPIDLY DETERIORATING WX CONDITIONS. IF RWY 18L HAD NOT BEEN AVAILABLE; OR IF THE INCIDENT HAD HAPPENED A MIN OR 2 LATER; I WOULD HAVE HAD TO CONSIDER GOING AROUND; INTO ADVERSE WX; TO AVOID THE WAKE TURB ASSOCIATED WITH B-757 OPS. FAA DIRECTS CTLRS TO PROVIDE A MINIMUM OF 4 MI SEPARATION FROM B-757 ACFT. THIS WAS NOT DONE.